1. Field of the invention
The present invention relates to multilayered structures comprising ethylene-vinyl alcohol copolymer (hereinafter referred to as EVOH), particularly to heat stretched multilayered structures comprising ethylene-vinyl alcohol copolymer and further to those heat stretched at a high speed, which does not, when heat stretched, form any pinholes, cracks, local thickness irregularities and the like and still has excellent gas barrier property.
2. Description of the prior art
EVOH has achieved much recognition as packaging film for foods and the like, particularly for foods requiring a barrier packaging film for oxygen, odor, flavor, etc. or for other contents requiring an odor-keeping packaging film. In these uses, EVOH is used in the form of multilayered structures comprising, besides itself, a thermoplastic resin layer(s), such as polyethylene, polypropylene, polystyrene, polyesters, polyamides and polyvinyl chloride, and a layer(s) of an adhesive resin represented by ionomers or ethylene-vinyl acetate copolymers.
Where a multilayered structure comprising EVOH and, among the above, polyester are heat stretched at a temperature lower than the melting point of EVOH, the EVOH layer is uniformly stretched without forming voids, cracks or local thickness irregularities, since polyester has a higher stretching tension than EVOH and hence the EVOH layer is influenced by the polyester layer. However, where a multilayered structure (film, sheet, parison and the like) comprising EVOH and polystyrene or the like is stretched into containers or the like at a temperature lower than the melting point of EVOH, the EVOH layer is not so affected by the polystyrene layer because of the stretching tension of polystyrene being smaller than that of EVOH, thereby forming therein a large number of microvoids, microcracks, local thickness irregularities and the like and hence greatly deteriorating the oxygen barrier property and appearance of the container formed. Such containers are therefore of no commercial value for packaging foods.
Several proposals have been made for the purpose of preventing generation of pinholes, cracks or the like in EVOH layer upon heat stretching. Namely, Japanese Patent Application Laid-open Nos. 88067/1978 and 20345/1984 disclose incorporation of various plasticizers into the EVOH layer and Japanese Patent Application Laid-open Nos. 141785/1977 and 36412/1983 disclose blending of polyamide resins with EVOH. However, the incorporation of a plasticizer is required to be in a large amount of 10 to 20 parts by weight based on 100 parts by weight of EVOH to sufficiently improve the heat stretchability, thereby raising problems of large decrease in the gas barrier property, decrease in the interlayer adhesive strength of the EVOH layer and the like, and is thus unapplicable in practice.
The blending of a polyamide resin is not applicable either for practical purpose, becasue the shaped articles obtained from such blend contain a large number of gels and/or suffer from serious coloring, due to, presumably, large chemical reactivity of the polyamide with EVOH. Even when a polyamide not causing formation of large number of gels or coloring is used, the obtained containers, which have apparently good appearance, show a large variability of gas garrier property values and are thus not reliable as a gas barrier container, due to, presumably, generation of minute pinholes at, particularly, high-speed heat stretching.
Accordingly, there has been desired development of an EVOH which gives highly reliable high gas-barrier containers, in other words, which does not, when the layer comprising it is heat stretched at a high speed, form minute pinholes, cracks, thickness irregularities and the like.
With respect to the specific polyester used in the present invention, Japanese Patent Application Laid-open No. 167617/1983 discloses the very polyester but does not describe at all about a composition of its blend with EVOH or a technical thought that the composition is improved in high-speed heat stretchability and in reliability in the gas barrier property. Further Japanese Patent Application Laid-open No. 211033/1986 discloses incorporating the specific polyester used in the present invention in EVOH and laminating the thus obtained composition directly on a polyester layer and that by doing so a firm interlayer adhesion is achieved. However, the patent application does not describe or anticipate that such incorporation and lamination produces improvement effect in stretchability and thermoformability, or about resulting improvement in the reliability of gas barrier property, of the obtainable multilayered structure, which is the very object of the present invention. Since such multilayered structure of polyester with EVOH as is described in the above patent application can generally be readily stretched at a low temperature of 80.degree. to 120.degree. C. and thermoformed at this temperature range, it is not necessary to modify EVOH to further improve the stretchability and thermoformability.
It thus was quite unexpectedly found that the EVOH composition of the above patent application improves the stretchability and thermoformability, and further the reliability of gas barrier property, of multilayered structures comprising the composition layer together with a layer of polystyrene, polypropylene or the like.